The poinsettia (Euphorbia pulcherrima) is a shrub belonging to the spurge or Euphorbiaceae family. It is native to southern Mexico and Central America, where it can reach 3 metres or more. Although poinsettias never grow this large in our homes, they are the most popular decorative plants during the holiday season. While poinsettias naturally bloom around Christmas in the wild, ornamental cultivars may bloom somewhat earlier.
The brightly coloured parts of a poinsettia are not the plant's flowers. They are actually bracts, or modified leaves surrounding the real flowers, which are tiny, yellow and not very showy. Poinsettias with red bracts have traditionally dominated the market, but today many different cultivars are available, offering consumers a wide range of colours and shapes, with pink, creamy white, yellow, burgundy, marbled, mottled or even wavy bracts.
What is commonly called the blooming period is the time when the bracts are coloured. This period may last for over four months under suitable conditions, whereas the real flowers are short-lived.
The bracts, or brightly coloured modified leaves, attract pollinating insects to a poinsettia's true flowers, which are not in themselves very showy. They are the tiny (4-5 mm) greenish-yellow buds in the centre of a bunch of 10 to 20 bracts. Poinsettias have a unique inflorescence, known as a cyathium in euphorbias. It is made up of a central female flower lacking petals of any kind, surrounded by male flowers reduced to a single stamen. This is enclosed in a cup-shaped ring of tiny incompletely fused bracts with one or more nectar glands on their sides.
Poinsettias produce flowers in response to day length, a physiological phenomenon called photoperiodism. In the wild, they flower in winter, when the days are short. To begin flowering, they need short days, or rather, long nights-at least 12 to 14 hours of uninterrupted darkness per day for 7 to 10 weeks, depending on the cultivar. While the real flowers fade after fertilization, the bracts, which are actually leaves, may remain colourful for months.